1. Field of the Invention
This invention relates to devices for producing a dense plasma at thermonuclear-fusion temperature and, in particular, to such devices where the plasma is produced in large volume with long containment time.
2. Discussion of the Prior Art.
Among various devices in use for producing hot plasmas in thermonuclear fusion research, a type of device called a plasma focus apparatus is the most successful one with respect to the Lawson criteria on n.tau., where n is the particle number density of a plasma and .tau., its confinement time. n.tau. of a plasma focus apparatus reaches 10.sup.12 which lies only two orders of magnitude below the value of the criteria for a sustained thermonuclear fusion. The operation and nature of the plasma produced by a plasma focus device are described in "Formation of a High-Density Deuterium Plasma Focus", by J. W. Mather, The Physics of Fluids, Vol. 8, No. 2, 1965, pages 366-377.
A schematic diagram of a typical plasma focus apparatus basically comprising two cylindrical electrodes is shown in FIG. 1.
Referring to FIG. 1, power is supplied by a capacitor bank of 10 kJ or more of energy storage at 18 kV or more of charging voltage. When the circuit is closed by a fast switch, an electric breakdown occurs at the base of the coaxial electrode along the surface of the insulator as indicated in FIG. 1. Subsequently, the current sheet is accelerated toward the end of the electrodes by J .times. B force where J is current density and B is magnetic induction. The pressure of the working gas and the length of the electrodes are initially adjusted so that the peak current appears when the current sheet reaches the end of the electrode. Due to the configuration of the conducting surface the current sheet collapses violently toward the axis of the electrode producing a dense and hot plasma, called a plasma focus, in a volume of .about.0.01 cm.sup.3 at a half inch above the center electrode. When the working gas is deuterium, d-d reactions take place in the plasma and 10.sup.9 -10.sup.11 neutrons per discharge are emitted. Typical plasma parameters are: particle density --10.sup.19 /cm.sup.3 ; plasma temperature --2 to 5 keV or 20 to 50 million degrees Kelvin; shape --1 mm diameter and a few cm long cylindrical form; sustainment time --typically 100 nanoseconds.
The plasma-focus apparatus currently is use has thus far no control over the duration of the sustainment (typically 100 nsec) and the size of the hot plasma (1 mm diameter). There has been an effort to scale up to neutron production in plasma focus devices by increasing input energy and the size of electrodes but results show no appreciable increase in both the sustainment time and the plasma volume. Use of a large electrode does not solve these limitations. A diameter of the center electrode up to 48 cm has been used but no appreciable changes on these limitations were possible. Further, due to its geometry it is difficult to form a system with two or more of such apparatus for higher performance.
As stated above plasma focus apparatus is the most successful one in approaching Lawson's criterion for thermonuclear fusion reaction. However, it is difficult to extend to a large-scale reactor development due to its geometrical limitations. Attempts to couple two plasma focuses have also been unsuccessful for devices of the type described in FIG. 1.